This invention relates generally to an infrared rays detecting apparatus, and more particularly to a cooling type infrared ray detecting apparatus which is cooled to a low temperature when in use.
Infrared rays detecting apparatus or infrared rays sensors are used in various fields such as meteorological observation by an artificial satellite, prevention of crimes, prevention of disasters, geological and resources investigations, medical treatment by infrared rays thermography and so forth because it can detect presence, a profile, a temperature, a composition and so forth of an object body without contacting the object body. Such infrared rays sensors are roughly classified into thermal sensors and sensors of the photoelectric effect type which make use of a semiconductor.
While thermal sensors generally have no wavelength dependency of sensitivity, they are low in sensitivity and also in speed of response, and accordingly, they are not suitably used as real time infrared rays sensors. On the other hand, while photoelectric effect sensors are high in sensitivity and speed of response, it is necessary to cool a sensing element or device substantially at a temperature of liquid nitrogen. Photoelectric effect sensors are classified into those of the photoconductive type, the photoelectromotive force type and the MIS type. Photoelectric effect sensors of the photoconductive type make use of a variation in resistance upon irradiation of light, and conventionally known sensors of the type just mentioned employ an infrared rays detecting device or element composed of a compound semiconductor crystal, for example, HgCdTe or the like.
An infrared rays detecting device of such photoconductive type is used in a condition that it is cooled substantially to a temperature of liquid nitrogen as described above in order to assure a high detection sensitivity. To this end, an infrared rays detecting apparatus is conventionally constructed such that a vacuum vessel having a high adiabatic characteristic is used and a device mounting portion in the vacuum vessel is cooled to a predetermined temperature by means of coolant such as liquid nitrogen or by means of a cryogenic cooling apparatus when the infrared rays detecting device is to be rendered operative.
An example of a conventional infrared rays detecting apparatus is described subsequently with reference to FIG. 1. A vacuum vessel 2 includes an outer tube 4 formed, for example, from KOVAR, and an inner tube 6 formed from KOVAR and glass joined together. As known in the art, KOVAR is an alloy of nickel, iron and cobalt. The spacing between the outer and inner tubes 4 and 6 is vacuum drawn to a very low pressure. A germanium window 8 through which infrared rays pass is provided at a top portion of the outer tube 4. Meanwhile, a device mounting portion 6a is provided at a top portion of the inner tube 6, and an infrared rays detecting device 10 formed, for example, from HgCdTe crystal and a temperature sensor 20 formed from a silicon diode or the like are provided on the device mounting portion 6a.
The device mounting portion 6a of the inner tube 6 is cooled to a very low temperature by means of coolant of liquid nitrogen or the like or by a cryogenic cooling device so as to assure a high sensitivity of the infrared rays detecting device 10. The infrared rays detecting device 10 is connected to a signal detection circuit 18 by way of a first pair of lead wires 12, a pair of terminals 14 and a second pair of lead wires 16. Meanwhile, the temperature sensor 20 is connected to a temperature control circuit 28 by way of a pair of lead wires 22, a pair of terminals 24 and another pair of lead wires 26. The temperature control circuit 28 controls the temperature of the device mounting portion 6a in response to a temperature detected by the temperature sensor 20 so that it may be kept at a fixed level in order to obtain an output of the infrared rays detecting device 10 corresponding to an amount of infrared rays incident thereon.
In this manner, the conventional infrared rays detecting apparatus controls the cooling temperature of the device mounting portion 6a such that it may be kept at a fixed level by way of the temperature sensor which is provided independently of the infrared rays detecting device 10. The temperature sensor 20 is formed from a material different from that of the infrared rays detecting device. However, in the conventional cooling temperature control wherein such a temperature sensor is employed, it is difficult to control the temperature of the mounting portion of the infrared rays detecting device strictly to a predetermined temperature, and the temperature of the detecting device mounting portion varies within a range of .+-.1.degree. C. or so with respect to the predetermined temperature. Since the cooling temperature has such a width of variation, characteristics such as resistance of the infrared rays detecting device are varied in response to the cooling temperature, and accordingly, there is a problem that a fluctuation takes place in an infrared rays image.